1. Field of the Invention
This invention relates to systems for positioning and mapping electrophysiology catheters and ablation catheters in the heart of a patient. The invention further relates to methods for error correction in electrocardiograph signals.
2. Background Art
U.S. Pat. Nos. 5,697,377 (the '377 patent) and 5,983,126 (the '126 patent) to Wittkampf disclose a system for determining the position or location of a catheter in the heart. The '377 patent and the '126 patent are hereby incorporated herein by reference in their entirety. In the Wittkampf system, current pulses are applied to orthogonally placed patch electrodes placed on the surface of the patient. These surface electrodes are used to create axis specific electric fields within the patient. The Wittkampf references teach the delivery of small amplitude, low current pulses supplied continuously at three different frequencies, one on each axis. Any measurement electrode placed in these electric fields (for example within the heart) measures a voltage that varies depending on the location of the measurement electrode between the various surface electrodes on each axis. The voltage across the measurement electrode in the electric field in reference to a stable positional reference electrode indicates the position of the measurement electrode in the heart with respect to that reference. Measurement of the difference in voltage over the three separate axes gives rise to positional information for the measurement electrode in three dimensions.
Although the Wittkampf system is both safe and effective there are several factors that can result in errors in the position of the measurement electrode. Some factors previously identified as sources of impedance modulation include the cardiac cycle and respiration. Both of these sources also cause actual physical movement of an electrode in addition to direct impedance effects. Mitigations to these modulators to enhance stability of electrode positional measurements include low pass filtering, cardiac cycle triggering, and respiration compensation. One factor not previously addressed is the tendency of biologic impedance to change over time. Changes in biologic impedance are attributable to changes in cell chemistry, for example, due to saline or other hydration drips in the patient, dehydration, or changes in body temperature.
If the biologic impedance changes over a longer term (i.e., minutes or hours), then apparent shifts of the measured locations of electrodes may occur. If an internal cardiac electrode is used as a reference electrode, these shifts may be negligible, since they are manifest as a scale factor change of only a few percent. For example, a 2 percent change with respect to a fixed reference 4.0 centimeters away will represent an error of 0.8 millimeters, which is generally considered acceptable. However, if it is desired to use an external body surface electrode as a fixed reference, and eliminate the requirement of a fixed intra-cardiac electrode reference, then 2 percent may represent an intolerable error source. For example, if the reference electrode is an “apparent” 40 cm from a mapping electrode, the error due to a 2% impedance drift would be 8 millimeters. The term “apparent” is used because while the actual distance to the reference electrode may be somewhat less, the intervening biologic of lung and muscle tissue is higher than that of blood, such that it scales to a larger distance.
The information included in this Background section of the specification, including any references cited herein and any description or discussion thereof, is included for technical reference purposes only and is not to be regarded subject matter by which the scope of the invention is to be bound.